It is often desirable in a louver installation to prevent to the utmost extent wind-driven rain water from passing through the louver (i.e., to prevent "water carry-over") from the outside environment into a duct or a space on the opposite side of the louver from the environment. Often, horizontal blade louvers of the drainable type are used, inasmuch as they have relatively low air flow pressure drops. In particular, they can be designed to minimize turbulence, a major source of pressure drop.
Vertical blade louvers can be designed to minimize water carry over by causing the flow to change direction as it passes through the louver and in so doing cause the rain drops to impinge upon and be captured by blade surfaces that generally face a portion of the flow path. One or more portions of the blades of vertical blade louvers can have projecting flanges to capture water driven along an adjacent surface and provide a vertical gutter along which the captured water flows to the sill. An unavoidable trade-off for inducing changes in the direction of flow through a louver is an increased pressure drop. In many situations, the trade-off is an acceptable one.